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You are on the right track in there being a possible vacuum loss. It is best to trace from the flapper actuators to the controls for the A/C-Heat then go from there to where they join at the brake booster. Somewhere along the way you have most likely lost the vacuum.
On the canistors you are referring to, can you take a picture of them? It has just been awhile since I woked on one of those vehicles.
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